Calcium hydroxyapatite, which has the formula Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.21 is the major constituent of bone and tooth mineral. Paul Brown in his paper Phase Relationships in the Ternary System CaO--P.sub.2 O.sub.5 --H.sub.2 O at 25.degree. C. teaches that hydroxyapatite should be viewed as a defect structure that exists over a compositional range Ca.sub.(10-x) (HPO.sub.4).sub.x (PO.sub.4).sub.(6-x) (OH).sub.(2-x), where x.ltoreq.1 and includes calcium deficient non-stoichiometric hydroxyapatite having calcium vacancies in the structure. J. Am. Ceram. Soc., 75 [1] 17 through 22 (1992).
Redpenning U.S. Pat. No. 5,310,464 describes the preparation by electrolysis of brushite coatings on metallic prosthetic appliances. The electrolyte is said to comprise an aqueous solution containing Ca.sup.2+ and dihydrogen phosphate ions. Brushite is calcium phosphate, dibasic and has the formula CaHPO.sub.4.2H.sub.2 0. Calcium phosphate ceramic coatings are said to accelerate bone fixation during the early recuperative stages after implantation of the prosthetic appliance.
Taylor et al. U.S. Pat. No. 5,330,826 describes depositing calcium phosphate ceramic coatings on metal substrates by electrolysis from an aqueous electrolyte solution that includes a salt of the anode metal. One example is cobalt sulfate. The cobalt metal is co-deposited with the calcium phosphate material and is said to secure the calcium phosphate material to the substrate and to increase the bond strength between the substrate and the calcium phosphate material. Nickel, chromium, or rhodium salts are disclosed for like anode materials.
Maybee et al. U.S. Pat. No. 5,338,433 describes codepositing a three component system of cobalt, chromium, and molybdenum to fix calcium phosphate materials to a substrate surface. A chelating element, EDTA, is used in the electrolyte to successfully electroplate cobalt and chromium simultaneously since the cobalt deposition rate would otherwise be too fast relative to that for chromium. The aqueous electrolyte provides ions of cobalt, chromium, and molybdenum from various soluble salts of these elements.
Klassen U.S. Pat. No. 5,458,863 describes a process for coating a metal substrate with hydroxyapatite that includes coating the metal substrate with brushite by electrolysis, periodically dislodging bubbles from the substrate during the electrolysis, and then converting the brushite coating to hydroxyapatite by immersing the metal substrate into an aqueous conversion liquor. The electrolyte for the brushite electrodeposition step is described as an aqueous solution of calcium phosphate, monobasic. The conversion liquor is said to comprise water and sufficient potassium hydroxide so that the pH of the liquor is about 10. At 80.degree. C., the conversion is said to be complete within 36 hours. Higher conversion temperatures are said to permit a shorter conversion time.
Hydroxyapatite coatings on medical implants have shown increased bone apposition in shorter periods of time than uncoated implants. A hydroxyapatite precursor, brushite, can be applied to metallic substrates of the type used for medical implants by several methods. Various physical and chemical vapor deposition techniques and electrolysis are described for this purpose. However, it would be desirable to provide faster, easier, milder, and more economical methods for coating prosthetic implants.